Arriving at the Tomb, Baal has already been freed by the Wanderer, and Duriel (Act II boss), a Great Evil, is left to guard it (it's speculated he was ordered to do such a lowly job as punishment by the Prime Evils for acting against them during the Dark Exile.

Defeating Duriel, and continuing in to the main chamber, Tyrael is found captive inside the tomb, who tells of what has happened, and urges the player to continue to Kurast, and Act III.

The second Act is a shorter length one with 19 total levels, but has a whooping 18 additional levels that are part of other levels (like Tal Rasha's Tomb Level 2-7 etc).

However, it's the only act which requires the player to finish several dungeons (to complete the staff) in order to finish the act. The other acts really only requires the player to finish the Act bosses.

Most of the quests are directly related to the progression of the story, and a lot more quests are mandatory compared to other Acts. The only mystery quest is Tainted Sun, which storywise might just be the Claw Vipers helping Diablo delay the hero, or might be related to something else completely.

Since we don't know the exact effects the Dark Wanderer has on the world around him, it's hard to know for sure.